The technical field of the invention is more particularly the field of fabricating and installing production risers for undersea extraction of oil, gas, or other soluble or meltable material or a suspension of mineral material from a well head that is underwater up to a support that is floating, in order to develop production fields installed in the open sea off-shore. The main and immediate application of the invention lies in the field of producing oil.
The technical sector of the invention is more particularly the field of undersea oil production in areas where oceanic and weather conditions are difficult or extreme, or indeed in Arctic or Antarctic areas, and using floating supports.
In general, an oil production floating support includes anchor means in order to enable it to remain in position in spite of the effects of currents, winds, and swell. It also generally includes means for drilling, storing, and processing oil, together with unloading means for take-off tankers, which calls at regular intervals in order to take off the production. The usual term for such floating supports or ships is “floating production storage off-loading” (with the initials “FPSO” being used throughout the description below) or indeed floating drilling & production unit (FDPU), when the floating support is also used for performing drilling operations using wells that are deflected in the depth of the water.
When sea and weather conditions, i.e. swell, wind, and currents are heavy, or indeed extreme, as in storms, it is preferable to anchor the FPSO via a turret, which is generally situated in conventional manner at the front or in the front third of the ship and on its axis, with the ship being free to turn about said turret in response to the wind, currents, and swell. Thus, wind, currents, and swell exert specific forces on the hull and the superstructures, with the FPSO being free to turn about the vertical axis ZZ′ and naturally taking up a position of the resistance. The pipes connecting the well heads are themselves connected in general to the underside of the turret and they are connected to the FPSO via a rotary joint incorporated on the axis of said turret, generally situated on the deck of said FPSO. When the weather conditions are likely to become extreme, as in the North Sea, in the Gulf of Mexico, or indeed in Arctic or Antarctic regions, the FPSO is generally disconnectable so as to be able to take shelter while waiting for acceptable operating conditions.
A floating production support having a system of anchor lines connected to the sea bottom and also bottom-to-surface connection pipes anchored to a turret comprises, in conventional manner:                a rotary device called a “turret”, comprising a cylindrical internal structure, preferably of circular section, within a cavity passing through the entire height of the hull of the floating support, said internal cylindrical structure being hinged to rotate relative to said hull by means of rotary guide elements, in particular by means of a rolling or friction bearing, and preferably a rolling bearing comprising wheels and guide tracks for said wheels, so as to allow said floating support to turn about a substantially vertical axis ZZ′ of said cylindrical internal structure and of said cavity, without leading to significant turning of said cylindrical internal structure about the same vertical axis ZZ′; and        a bottom mooring structure enabling firstly said anchor lines to be moored and secondly first flexible pipes connected to undersea pipes resting on the sea bottom to be connected directly or via bottom-to-surface connection pipes, said mooring structure being arranged under the hull of the floating support, being fastened to an underwater bottom wall of said cylindrical internal structure, and thus secured to said cylindrical internal structure, in such a manner that the mooring lines and said flexible pipes that are connected thereto are not caused to turn when said floating support is itself caused to turn; and        said flexible pipes connected to said first flexible pipe or in continuity therewith rising within the cavity up to a connection of the type referred to as a rotary connection having a first portion that is secured to the deck of said hull that is connected to a plurality of pipes or lines on the deck, and a second portion secured to the top platform of said cylindrical internal structure situated above the level of the sea surface, the top ends of said flexible second pipes rising within said cylindrical internal structure of the turret being connected, generally via pipe bend elements, to said second connection portion, said first connection portion secured to the floating support on the deck of the floating support being hinged to rotate relative to said second portion of the connection secured to said cylindrical internal structure via a rotary connection so as to allow said floating support to turn without turning the ends of said second flexible pipes within said cylindrical internal structure.        
The rolling bearing is located either level with the deck of the floating support, or else in a low portion in a wet zone, i.e. the bearing is underwater, or indeed it comprises a combination of both of the above configurations.
Said bottom wall of said cylindrical internal structure is preferably assembled thereto in leaktight manner so as to avoid the underwater portion of the inside of said internal structure being invaded by sea water. This waterproofing makes maintenance operations and other actions carried out by personnel inside the turret easier to perform, in particular in order to make connections to the flexible pipes. However, and above all, this sealing enables Archimedes' thrust to apply to the volume displaced by the cylindrical internal structure of the turret and lightens in part the vertical forces generated by the anchor lines, by said flexible pipes at the guide elements, and compensates for the weight of said cylindrical internal structure.
Numerous bottom-to-surface connection configurations have been developed, in particular in the Applicant's patents WO 2009/122098, WO 2009/122099, and WO 2010/026334, which describe an FPSO provided with such a turret and the associated flexible pipes, more particularly for use in the extreme conditions to be encountered in the Arctic. Such a configuration is advantageous for medium depths of water, i.e. in the range 100 meters (m) to 350 m, or indeed 500 m to 600 m. In particular, using flexible pipes over the full height of the water between the rigid undersea pipes resting on the sea bottom and the floating support allows the floating support to move to a greater extent than is possible when rigid pipes are used.
When the depth of water reaches 1000 m to 1500 m, or indeed 2000 m to 3000 m, installing flexible pipes over the entire depth of the water becomes very expensive since flexible pipes are very complex and difficult to fabricate, and it is preferred to use bottom-to-surface connections of the so-called “hybrid” tower type, in which the substantially vertical portion of the tower is constituted by rigid pipes between the sea bottom up to a depth of about 50 m to 100 m below the water surface, with the top ends of the rigid pipes then being connected to the FPSO via flexible pipes having a length of 150 m to 350 m and in the configuration of a catenary or a dipping double catenary. Numerous patents in the name of the Applicant show advantageous arrangements for making this type of bottom-to-surface connection, and in particular WO 2011/144864 in the name of the Applicant.
An oil field is generally constituted by a plurality of well heads situated at various distances from the FPSO, which may be as much as several kilometers, each of them being connected by pipes that may be rigid, or flexible, by umbilicals, and by electric cables. In general, the bottom-to-surface connection of a said well head comprises at least one pipe for producing oil or for injecting water or gas, plus an umbilical for controlling the well head, and possibly also one or more electricity cables. These rigid or flexible pipes, umbilicals, and cables are laid on the sea bottom beside one another and must not overlap along their length. Thus, the arrangement of the flexible pipes under the turret and the way they are connected to the turret depend on the layout of the well heads and of the undersea pipes resting on the sea floor and constituting the layout of the field. Furthermore, in certain circumstances, a large number of bottom-to-surface connections are required, and in order to avoid interference between said pipes in the depth of water, said flexible pipes are arranged side by side and close to one another, possibly in one or more circles, and in the same order as the ends of the pipes resting on the sea bottom, one beside another on the sea bottom 10.
Patent WO 2011/061422 A1 in the name of the Applicant describes an FPSO that is not provided with a turret, but that is anchored in permanent manner, with a north heading, on 16 anchor lines, with a plurality of bottom-to-surface connections coming from the west being connected all along the side of the vessel, thereby providing a large concentration of pipes coming in a single direction.
In certain oil field configurations, it can thus happen that the periphery of the turret, in particular the periphery of the bottom mooring structure and/or the bottom wall of said cylindrical internal structure presents a high concentration in a small region of flexible pipes of large diameter, and thus of large weight per unit length, that are all connected over a narrow angular zone of the periphery, whereas in a diametrically opposite zone, there are to be found only flexible pipes or cables of low weight per unit length, or indeed no pipes in certain circumstances. FIG. 4, described below, illustrates this type of arrangement.
However, a flexible pipe in suspension that is connected to the turret exerts tension thereon, which the turret takes up via the pipe's connection to said turret, which tension generates large vertical forces (force of magnitude F), and also lesser horizontal forces (force of magnitude H) where the pipe is connected to the turret, in particular for a bottom-to-surface link of great depth. Under such circumstances, a distribution of flexible pipes that is irregular and/or asymmetrical leads to force variations and thus to stress variations that are large all along the periphery of the turret, and these are transferred and taken up in non-uniform manner in particular via said rotary guide element. Furthermore, in an off-shore oil field there is generally a predominant direction for swell, winds, and currents, and when the floating support pivots about the turret under the effect of the swell and/or the wind or sea current, most movements are restricted in general to a substantially constant angular sector, e.g. ranging from north to southwest. Said rotary guide elements are then subjected to alternating forces that are localized in even more non-uniform manner around the periphery of the turret for this additional reason, thereby giving to increased or accelerated localized wear of said rotary guide elements.